Antioxidative, Hemocompatible, Fluorescent ... - ACS Publications

Article pubs.acs.org/JAFC

Antioxidative, Hemocompatible, Fluorescent Carbon Nanodots from an “End-of-Pipe” Agricultural Waste: Exploring Its New Horizon in the Food-Packaging Domain Manashi Das Purkayastha,† Ajay Kumar Manhar,‡ Vijay Kumar Das,# Anjan Borah,† Manabendra Mandal,‡ Ashim Jyoti Thakur,# and Charu Lata Mahanta*,† †

Department of Food Engineering and Technology, School of Engineering, ‡Department of Molecular Biology and Biotechnology, School of Science, and #Department of Chemical Sciences, School of Science, Tezpur University, Assam, India 784028 S Supporting Information *

ABSTRACT: The attention of researchers is burgeoning toward oilseed press-cake valorization for its high protein content. Protein removal from oil-cakes generates large quantities of fibrous residue (oil-and-protein spent meal) as a byproduct, which currently has very limited practical utility. In the wake of increasing awareness in waste recycling, a simple environmentally benign hydrothermal carbonization process to convert this “end-of-pipe” waste (spent meal) into antioxidative, hemocompatible, fluorescent carbonaceous nanoparticles (FCDs) has been described. In the present investigation, an interesting application of FCDs in fabricating low-cost rapeseed protein-based fluorescent film, with improved antioxidant potential (17.5−19.3-fold) and thermal stability has been demonstrated. The nanocomposite film could also be used as forgery-proof packaging due to its photoluminescence property. For assessing the feasibility of antioxidative FCDs in real food systems, a comparative investigation was further undertaken to examine the effect of such nanocarbon-loaded composite film on the oxidative shelf life of rapeseed oil. Oil samples packed in nanocomposite film sachets showed significant delay in oxidative rancidity compared to those packed in pristine protein-film sachet (free fatty acids, peroxide value, and thiobarbituric acid-reactive substances reduced up to 1.4-, 2-, and 1.2-fold, respectively). The work presents a new concept of biobased fluorescent packaging and avenues for harnessing this potent waste. KEYWORDS: rapeseed press-cake, carbon nanodots, hydrothermal carbonization, nanocomposite film, rapeseed protein

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INTRODUCTION Currently, fluorescent nanoparticles (NPs) (also known as quantum dots (QDs) due to their typical sizes below 10 nm), a young smart member of the nanomaterial family, is gaining attention, especially in the fields of optoelectronics, bioimaging, biomedicines, etc. Unfortunately, application of NPs in foods1−5 and their packaging materials is still in its infancy. The reason behind this seems to be the raised toxicity concern and environmental hazard of the fluorescent semiconductor QDs, which are based on metallic elements and heavy metals (elements from the periodic groups II−IV, III−V, and IV−VI). Alternatively, fluorescent carbon NPs (called fluorescent carbon dots or FCDs or C-dots or simply CNPs) can become an exciting option, because they are greener for the environment and show aqueous dispersibility, numerous possible applications in nanobiotechnology, and far less toxicicty to living organisms. Recently, Sk et al.6 reported the presence of FCDs in regular carbohydrate-based food items such as bread, jaggery, sugar caramel, cornflakes, and biscuits and also showed their biocompatibility. This novel discovery alleviated the misapprehension that all fluorescent NPs are toxic and revealed that humans have been consuming fluorescent nanomaterials in the form of food caramels for centuries, and thus they can be considered safe. Recently, procurement of novel C-dots from greener sources such as egg,7 orange juice,8 banana juice,9 soy milk,10 glucose, sucrose, starch,11 citric acid,12 chitosan,13 etc., has been the © 2014 American Chemical Society

center of attraction. Although they are the current state of the art, most of these methods suffer to some degree from certain drawbacks such as the requirement of sophisticated and expensive equipment such as a laser beam, microwave, or high-power autoclave; neat chemicals as precursors for synthesis; or even multisteps of operation along with surface passivation to improve the water solubility and photoluminescent properties of C-dots. Moreover, use of such food/feed or other edible stuffs as raw materials for FCD preparation is expensive for bulk use, and in the long run would create undue strains on the food resource system and food security scheme. This is a major drawback to the commercial synthesis of NPs from these edible resources, which is unlikely to be extended in the near future. This motivated us to search for a simple economical technique for the production of FCDs from industrial or agricultural byproducts. This is a challenging but worthy concept as the use of waste materials is one of the most attractive options to reduce the raw material cost and also seems benign from an ecological point of view. In this context, very few papers are available. Lu et al.14 reported such an endeavor from pomelo peel, and the resulting CNP was used for the detection of mercury ions in water. Wang et al.15 Received: Revised: Accepted: Published: 4509

January 9, 2014 April 28, 2014 May 1, 2014 May 1, 2014 dx.doi.org/10.1021/jf500138f | J. Agric. Food Chem. 2014, 62, 4509−4520

Journal of Agricultural and Food Chemistry

Article

rization of the complete investigation is depicted pictorially in Supplementary Figure S1 in the Supporting Information.

prepared C-dots from egg-shell membrane and used it for designing a fluorescent probe for glutathione detection. In a closely related work, FCDs were prepared from used coffee grounds,16 and their practicality in cell imaging and detection of angiotensin I and insulin was assessed. Most reported C-dots have been prepared for bioimaging, which limits their application in other fields. As such, it has become urgent to develop effective routes to create functional C-dots as well as expand their applications.10 Explicitly, the role of FCDs in the food packaging domain is unexplored to date. Industrially, mechanical pressing of the oilseeds produces the oil (main product) and the press-cake (byproduct). Due to the high quantity of protein, the oil-cake or meal is being used mainly as animal feed. Recently, owing to the high nutritive value of the meal protein, techniques are now being devised to harness it. After the extraction of meal protein, the residual fibrous waste material (protein spent meal) is discarded and is not extensively used in industries. At present there are very few possibilities for the utilization of this waste; usually the residue is disposed of as landfill and, hence, is as an “end-of-pipe” waste. In this work, the synthesis of green fluorescent quantumsized carbonaceous NPs from “oil-and-protein spent” rapeseed meal by a facile hydrothermal process has been reported for the first time. These NPs are referred to as “carbogenic” because of their high oxygen content along with carbon.12 Undeniably, the most active area of food nanoscience research and development is “packaging”. This is likely connected to the fact that the public has been shown in some studies to be more willing to embrace nanotechnology in “out-of-food” applications than those in which NPs are directly added to food.17 Therefore, the practicality of the synthesized FCDs for making green-fluorescent edible biopackaging material was investigated in this study, thereby expanding the potential application of C-dots. Synthetic plastic packaging is beginning to be replaced by biodegradable ones because of environmental concerns. Films made of carbohydrates and protein are long and have empirically been used to make food grade biodegradable packaging materials. A recent approach to this technology involves the use of vegetable protein from different types of oil-cakes such as cottonseed,18 pumpkin seed,19 soybean,20 and rapeseed/canola.21 In this milieu, edible films using rapeseed protein were developed in the current investigation, because this biopolymer can be obtained as a value-added product from the underutilized meal. As such, the study was extended to evaluate the effect of such a nanoadditive on the physicochemical properties of rapeseed protein film. Mostly, consumers have to rely on holograms and other displays on the packets to segregate authentic products from their duplicate inferior counterparts. Often, fluorescent dyes are incorporated into the packaging to help customers easily detect authentic products (e.g., erythrosine); however, many of these dyes have been shown to be cytotoxic to a variety of mammalian cell types.22 The unique “green” photoluminescence property of the here-in synthesized carbogenic NPs, along with high antioxidative potential and hemocompatibility, offer a solution to this problem. It is envisioned that FCDincorporated edible films would not only be useful for quality control to ensure that consumers are able to purchase authenticated products, but would also improve the oxidative stability of the produce/commodity. To justify this possibility, further study was conducted to see the effect of FCD−protein composite film on the oxidative shelf life of an oil sample. For simplicity, the preparative protocol along with the summa-

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MATERIALS AND METHODS

Chemicals. All solvents and reagents were obtained from E. Merck (India), of either high-performance liquid chromatography (HPLC) grade or analytical reagent grade, and were used without further purification. Materials and Sample Preparation. Cold-pressed rapeseed oil and press-cake were obtained from Assam Khadi and Village Industries Board, Guwahati, India. Press-cake was detoxified prior to protein extraction according to the reported procedure23 and then stored at −20 °C until use. Protein Extraction from Detoxified Meal. Aqueous suspension of detoxified meal was prepared with water (30:1 v/w), followed by the addition of 0.1 M NaCl and 0.4% sodium sulfite. The pH of the suspension was adjusted to 11 ± 0.1 with 1 N NaOH solution; the suspension was mixed (200 rpm) for 1 h at 25 °C in the orbital shaker (Sartorius Stedin Biotech, CERTOMAT IS), followed by centrifugation (Sigma 3-18K centrifuge) at 7000 rpm for 20 min at 4 °C. The solid residue (oil-and-protein spent meal) was vacuum-dried (lab companion model OV-12, Jeiotech Co., Korea) for 48 h at 40 °C, ground to pass through a sieve having 250 μm pores, and then stored at −20 °C for further use. On the other hand, the protein-rich supernatant was filtered through Whatman filter no. 41, and then ammonium sulfate was added up to 85% saturation. The mixture was kept in an ice bath for 3 h with gentle stirring and then centrifuged at 10000 rpm for 20 min at 4 °C. The obtained protein precipitate was redispersed in Milli-Q water (Millipore Water Purification System, Model-Elix, USA), neutralized to pH 7, dialyzed against water at 4 °C, and finally freeze-dried, which was subsequently used for making the composite film. Synthesis of C−Dots from Oil-and-Protein Spent Meal. FCDs were synthesized by hydrothermal carbonization (HTC) of spent meal using the reported protocols8,24 with slight modification. In a typical procedure, oil-and-protein spent meal (10 g) was dispersed in 1 N NaOH solution (solvent) at a solvent/meal ratio of 463:1 (v/w). The suspension was refluxed in an oil bath under magnetic stirring at 180 °C for 3 h (details of the optimal production of FCDs using response surface methodology are given in supplementary section 1 in the Supporting Information). After the reaction is over, the resultant black solution was allowed to cool and then centrifuged at 3000 rpm for 10 min to separate the unreacted residue. The brownish supernatant was washed with dichloromethane to remove the unreacted organic moieties. Subsequently, the aqueous phase was mixed with acetone (water/acetone ratio =1:3 v/v), and the acetonic extract was finally reduced to dryness under vacuum. The residue was dispersed in MilliQ water, dialyzed against water (using 1 kDa membrane, Himedia, India), and finally dried under vacuum at 40 °C to obtain FCDs (≈0.942 g). Characterization of C−Dots. High-resolution transmission electron microscopy (HRTEM; JEOL, JEMCXII) images along with the selected-area electron diffraction (SAED) pattern were obtained at an accelerating voltage of 200 kV, and the sample was prepared by drop casting 2 μL of NP solution (0.25 mg/mL) on a 300 mesh carbon-coated copper grid and subsequent air-drying before analysis. X-ray dif f raction (XRD) measurement was carried out in thin film mode of powdered sample using a Rigaku Miniflex model (Japan), operated at 30 kV voltage and a current of 15 mA with Cu Kα radiation source (λ = 1.54 Å) at a scan rate of 5° (2θ) min−1 over the range of 10−70°. Thermogravimetric analysis (TGA) curves were collected on a TG 50 model (Shimadzu, Japan). The samples were combusted under nitrogen flow (10 mL/min, to avoid thermo-oxidative reactions) at temperatures ranging from 25 to 600 °C, at a rate of 3 °C/min. Fourier transform infrared (FTIR) spectra were obtained on a FTIR Nicolet Magna 5PC spectrometer (Impact-410, Madison, WI, USA), coupled to a PC with Omnic analysis software and having a deuterated triglycine sulfate (DTGS) detector and Nernst filament as the IR light 4510

dx.doi.org/10.1021/jf500138f | J. Agric. Food Chem. 2014, 62, 4509−4520


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buffy coat. RBCs were washed three times with PBS (pH 7.4) and resuspended in the same buffer to make a packed cell volume of ≈10% (w/v) as stock. Then, cell labeling was carried out by mixing different aliquots of the above RBC stock suspension with different volumes of FCDs (30.4, 45.5, 60.8, and 75.8 μL) from its stock solution (660 mg/ mL) for making the final concentrations of FCDs as 20, 30, 40, and 50 mg/mL by addition of PBS in each set of experiments. Afterward, the erythrocytes were incubated for 4 h at room temperature, centrifuged (3000 rpm, 15 min) at 4 °C, and then washed three timeswith PBS. Finally, 20 μL of the cell suspension was taken and used to prepare a smear on glass slides, which were viewed under a fluorescent microscope (LEICA DM 3000, power = ebq 50 qc, USA) attached to a LEICA DFC 450C camera. Preparation of Rapeseed Protein−FCD Composite Film and Its Application for Oil Packaging. Rapeseed protein film solution was prepared by using the methodology reported by Cho and Rhee.20 Freeze-dried rapeseed protein powder (8 g) and glycerol (4 g) were dissolved in 100 mL of Milli-Q water. The suspension was heated on a hot-plate magnetic stirrer for 10 min at 80 °C until a homogeneous clear solution was obtained. FCDs (20 or 30 mg) were added to the mixture, and blending was further continued for 15 min at 80 °C. The mixture (40 mL each) was cast onto glass Petri plates and then dried in an oven at 35 °C for 48 h. Films were conditioned in an environmental chamber (plant growth chamber HB 303DH, K&K Scientific Supplier, India) set at 25 °C with 50% relative humidity (RH) for 48 h. The composite films with 20 and 30 mg of FCDs/100 mL of film-forming solution were accordingly coded F1 and F2, respectively. Film without FCDs served as control and was designated F0. Films were drawn into small rectangular sachets/pouches (5 cm × 3.5 cm), each of which was filled with 4 mL of cold-pressed rapeseed oil, and then sealed after the headspace had been flushed with nitrogen gas. Oil-containing sachets were stored for 28 days in the environmental chamber (25 °C, 50% RH), and the oil samples were analyzed after every 7 days. Oil samples packed in F0, F1, and F2 were accordingly labeled S0 (control), S1, and S2, respectively. Characterization of the Composite Films. Film thickness was measured at five random positions with a micrometer (Mitutoyo Corp., Japan). The mean thickness was used to calculate the mechanical and barrier properties of films. For moisture content determination, film samples were weighed into aluminum pans and dried at 105 °C in an oven for 24 h (until the equilibrium weight). The weight loss of the sample was determined, from which the moisture content was calculated. Water vapor permeability (WVP) was determined gravimetrically following the standard method of the American Society for Testing and Materials (ASTM).26 Conditioned film samples were sealed to glass cups (4.7 cm diameter) containing water. The film-covered cups were placed in the environmental chamber (25 °C, 75% RH). Cups were weighed periodically using an analytical balance (Denver Instrument, Bohemia, NY, USA), until steady state was reached (±0.0001 g). Once the steady state was reached, the water vapor transmission rate (WVTR, ng/m2 s) of the film (eq 3) was determined from the slope obtained from the regression analysis of moisture weight gain (Δw) transferred through the film area (A) during a definite time (Δt). WVTR was then used to calculate WVP using eq 4.

source. The sample was ground with KBr powder to form well-defined pellets for IR measurement, with 32 scans from 4000 to 400 cm−1 at a resolution of 4 cm−1. Micrographs of the sample were obtained using a JSM-6390LV scanning electron microscope (SEM; JEOL, Japan) at an accelerating voltage of 15 kV. Prior to SEM observation, samples were mounted on stubs with double-sided adhesive tape, followed by coating the samples with a thin layer of gold. For determining the elemental composition and purity, sample was prepared on a carbon-coated copper grid and kept under vacuum desiccation for 3 h before they were loaded onto a specimen holder. Elemental analysis on single particles was carried out using electron dispersive an X-ray spectroscopy (EDX, JSM-6390LV) attachment equipped with SEM. 1 H and 13C nuclear magnetic resonance (NMR) spectra were detected at 400 MHz by a JEOL NMR system (Japan), using the inbuilt DELTA (δ) software (version G4.3.6, Japan) provided by the manufacturer. The ultraviolet−visible (UV−vis) absorption spectrum of aqueous NP solution (0.1 mg/mL) was recorded on a UV−vis spectrophotometer (CECIL 7400, 7000 series, Aquarius). The photoluminescence (PL) spectrum of aqueous NP solution (0.1 mg/mL) was measured on a photoluminescent spectrophotometer (model LS 55, PerkinElmer, Singapore PTE Ltd., Singapore). Illumination of the aqueous NP dispersion for detecting its fluorescence property was done inside a UV cabinet (BD-198 model, Test Master, Kolkata, India). Quantum yield (QY) of C-dots was measured in reference to quinine sulfate in 0.1 M H2SO4 (QY = 58% at 354 nm excitation).6,25 The formula used for QY is (QY)Sm = (QY)St × [(PL area/OD)Sm /(PL area/OD)St ] 2 × ηSm /ηSt2

(1)

where Sm indicates the sample, St indicates the standard, η is the refractive index of the solvent, and PL area and OD are the fluorescence area and absorbance value, respectively. For aqueous solutions ηsm/ηst = 1 was chosen.6 Antioxidant activity was measured by using the modified 2,2diphenyl-1-picrylhydrazyl (DPPH) radical method.26 Briefly, 3 mL of sample solution was mixed with 1 mL of 1 mM methanolic solution of DPPH. The mixture was then vortexed and incubated in the dark at ambient temperature for 30 min. The absorbance was measured at 517 nm in a UV−vis spectrophotometer. In the case of FCDs or ascorbic acid (as standard), the amount was varied from 10 to 180 μg/mL of reaction mixture,27 whereas for film samples, 25 mg of each film was dissolved in 3 mL of Milli-Q water.26 The DPPH scavenging percentage was calculated using the formula

scavenging activity(%) = {(AD − A S) × 100}/AD

(2)

where AD and AS are the absorbance of the DPPH solution and the standard/sample, respectively. The hemolytic activity assay was performed according to the reported procedure.23 Briefly, fresh goat blood from a slaughterhouse was collected in a centrifuge tube containing anticoagulant, trisodium citrate (3.2%), and was centrifuged at 3000 rpm for 10 min. The supernatant was discarded, and only the red blood corpuscles (RBCs or erythrocytes) were collected. RBCs were further washed three times with phosphate buffer solution (PBS, pH 7.4). A 10% (v/v) suspension of erythrocytes in PBS was prepared, and 1.9 mL of this erythrocyte solution was placed in a 2 mL centrifuge tube; 0.1 mL of FCDs in PBS was added to it. The tubes were then incubated for 1 h at 37 °C. For comparison, Triton X-100 (0.2%) and PBS were taken as positive and negative controls, respectively. After incubation, the tubes were subjected to centrifugation at 3000 rpm for 10 min, and finally the absorbance of the supernatant was taken at 570 nm in the UV− visible spectrophotometer. Cell labeling with FCDs was done according to the procedure of Chandra et al.28 The erythrocytes enriched fraction was centrifuged twice (3000 rpm, 15 min) at 4 °C to remove the residual plasma and

WVTR = Δw/A(Δt )

(3)

WVP = WVTR(x /Δp)

(4)

x is the average thickness of the film, and Δp is the partial water vapor pressure between the two sides of the film. Oxygen permeability of the films was determined by using the wet chemical procedure of Ayranci and Tunc.29 Tensile strength (TS, in MPa) and Elongation at break (EB, %) of the film was measured according to ASTM D-882-91.30 For evaluating puncture strength (PS, given as force in Newton (N)), film was punctured with a 2 mm probe in a Texture Analyzer (TA HD Plus, Stable Micro Systems, UK). 4511



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Figure 1. (a) TEM image of FCDs thus formed; (b) corresponding HRTEM image of two nanoparticles (inset, pictorial representation of the morphology of one nanoparticle); (c) SEM image of the clusters of nanoparticles obtained after drying; (d) SAED image; (e) XRD pattern; (f) TGA curve of FCDs.

Figure 2. (a) FTIR spectrum, (b) EDX data, (c) 1H NMR, and (d) 13C NMR of FCDs. Color intensity of the film was measured using a Hunter Lab Colorimeter (Ultrascan, VIS-Hunter Associates Lab., USA), fitted with a large area port (2.5 cm diameter aperture). The instrument (including 65°/0° geometry, D25 optical sensor, 10° observer, specular light) was calibrated using white and black reference tiles provided by the manufacturer. For determining opacity, film absorbance was measured at 600 nm using a UV−vis spectrophotometer.26 The sample was cut into a rectangle piece and directly placed in a spectrophotometer test cell, using an empty test cell as the reference. The opacity index of the film was calculated by using the equation

opacity = Abs600 /x

(5)

where Abs600 is the absorbance at 600 nm and x is the average film thickness (mm). Analytical Tests for Oil Sample. Free fatty acid (FFA) and peroxide value (PV) were determined according to the method of Chaijan et al.31 and expressed as grams per 100 g of lipid and milliequivalents (mequiv) of free iodine per kilogram of lipid, respectively. Conjugated diene (CD) was estimated from the absorbance value at 233 nm using cyclohexane as the solvent blank.32 4512



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Figure 3. (a) UV−vis spectrum; (b) PL spectrum of aqueous dispersion of FCDs (0.1 mg/mL); (c) photographs of aqueous FCD dispersion (0.25 mg/mL) observed under white light and the same under UV light; (d) gradual bleaching of DPPH solution by FCDs; (e) DPPH scavenging activity of FCDs in comparison to that of ascorbic acid. For thiobarbituric acid-reactive substances (TBARS) assay,31,33,34 oil sample (0.5 g) was mixed with 2.5 mL of a solution containing 0.375% thiobarbituric acid, 15% trichloroacetic acid, and 0.25 N HCl. The mixture was heated in a boiling water bath (95−100 °C) for 10 min to develop a pink color, cooled with running tap water, and then centrifuged (3600g) at 25 °C for 20 min. The absorbance value of the supernatant at 532 nm was expressed as the result (A532). Statistical Analysis. All analyses were performed in triplicate, and the mean value was calculated. Analysis of variance (ANOVA) and separation of means were carried out by Tukey test, using SPSS software (version 16.0, SPSS Inc., Chicago, IL, USA) and considered significantly different at p < 0.05.

The thermal behavior of FCDs was investigated by TGA. The thermogram (Figure 1f) exhibited a two-step degradation pattern with the initial degradation (5% indicates that the test material causes damage to RBCs; this criterion was exceeded at the FCD concentration of 50 mg/mL. Hence, FCDs were found to exhibit hemolytic activity only at higher concentration (≥50 mg/mL), which is much lower than that reported for their other counterparts (fullerenes, carbon nanotubes, etc.) and several similar NPs.41 This benevolent feature may be due to the presence of congenial surface functional groups, making FCDs tolerable at low concentration. Yildirim et al.40 also found that surface functionalization of nanostructures can render them completely nonhemotoxic. To get a closer insight into the FCD-induced biological effect on RBC, the uptake of FCDs by the RBCs was examined by fluorescence microscope (Figure 5c). With increasing dosages (20, 30, and 40 mg/mL), no marked changes in the 4515



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Figure 6. Photograph showing the pristine protein film and the nanocomposite films under normal light (on left) and UV light (on right) (F0 (a), F1 (b), F2 (c)) and their corresponding SEM images (F0 (d), F1 (e), F2 (f)).

Figure 7. (a) FTIR spectra and (b) TGA curves of F0, F1, and F2.

(FCDs) in the matrix was found. FCDs appeared to be embedded on the polymeric matrix and exhibited a heterogeneous size distribution most likely due to some agglomeration. Such problems of agglomeration of nanomaterials in composite films have often been reported.44 Agglomeration usually occurs when a relatively high loading of the nanomaterial is added to the matrix, as is the case here. Apparently, such tiny clustering did not affect the green fluorescence of the films. The effect of the addition of FCDs into protein film was initially evaluated by FTIR. When different compounds are mixed, physical bonds and chemical interactions are reflected by changes in characteristic spectral peaks.42 FTIR spectra of F0, F1, and F2 are shown in Figure 7a. The main broad peaks are maintained, the spectra of the films with FCDs being similar to that of the control. Special mention should be made of the peaks between 3500 and 3000 cm−1, corresponding to the hydroxyl groups or hydrogen bonds or to the N−H group, that are weaker in F0 when compared to F1 or F2. This stretch appeared to be more recognizable with increasing FCD concentration. This indicates higher amounts of O−H or N−

brown, which might be the cause for dark coloration. As anticipated, F0 did not show any green glow under UV light, whereas under the same condition, F1 and F2 appeared as intense green-light-emitting films. From this it is clear that a homogeneous dispersion of the nanocomponent in the film matrix is obtained, without the presence of any visible greencolored fluorescent patches or clusters. Thus, we were successful in fabricating fluorescent glowing edible films based on the luminescent C-dots, which is envisioned to be used in forgery-proof packaging. It is expected that such packaging would benefit consumers, industry stakeholders, and food regulators. At this point, one of the objectives in this work was accomplished, and the comparative study of the physicochemical properties of these films was undertaken. SEM was used to characterize the topography and morphology of the films (Figure 6d−f). The microstructure of F0 presented rough ridge-like protrusions on the surface, typical of protein films, as has been also reported for films prepared with soy protein,43 rapeseed protein,21 and pumpkin seed protein.19 Surprisingly, F1 and F2 did not show any such ridge structures; rather, a good distribution of the additive 4516



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H groups in F1 and F2 than in F0. Derived from this fact, it is feasible to believe that FCDs, with hydrophilic surface moieties, may form hydrogen bonding between themselves or with amino groups of protein, especially when the protein is partially unfolded (denatured) during heating of a film-forming solution. This might be one of the possible reasons that F1 and F2 had more compact, uniform, and homogeneous surface morphologies compared to F0, as viewed under SEM (Figure 6d−f), mainly due to high compatibility of FCDs in the protein matrix. Addition of FCDs led to the emergence of a few new peaks in the region of 717−727 cm−1 (ascribable to out-of-plane C−H bending in aromatic substitution) and 878−881 cm −1 (correlating to the presence of vinyl groups). Moreover, in F1 and F2, the intensity of C−H or C−N stretching at 1400− 1450 cm−1 increased and seemed to be more distinguishable than that in F0. All of these facets signify the possibility of new interactions and cross-links between FCDs and polymeric matrix. The thermal stability of the composite films was tested by TGA. The thermograms in Figure 7b show that all of the film samples follow a similar degradation pattern, indicating uniform dispersion and high interactions of FCDs with the film (polymer) matrix.45 Surprisingly, in all three samples, no significant mass loss was noted up to ∼260 °C, beyond which decomposition of glycerol and protein occurred.18 The char yield is the nonvolatile carbonaceous material generated on pyrolysis, which is indicated by the residual weight after the decomposition step. It is worth mentioning that the increase of the residual yield (the char) with the increase of FCD concentration was detected, which points to the thermoinsulating nature of the C-dots as a nanofiller for the polymer matrix, a property similar to that of carbon nanotubes.46 After degradation, the net weight loss of F0 was found to be ∼80.58%, whereas those of F1 and F2 were ∼59.73 and ∼45.65%, respectively. Thus, with the increase in FCD content, significant (p < 0.05) reduction in weight loss was observed, thereby proving FCDs as potent thermo-insulators for proteinaceous films, a desirous property for high-temperature processing. Adding FCDs to the film matrix had little or no influence on film thickness (p > 0.05) (Table 1). This might be due to their small size. Although the same amount of film-forming solution was used, the slightly higher thickness of F0 may be due to its ridges-like rough outgrowths on its surface, as seen under SEM. The presence of FCDs led to an increase of water affinity of edible films, showing that moisture content increases in F1 and F2 significantly (p < 0.05) as compared to F0 (Table 1). This may be due to the hydrophilic behavior of the −OH groups in FCDs, which probably influenced WVP, increasing the adsorption of water molecules42 in FCD-incorporated films when compared with F0. As a consequence, the water vapor barrier property of F1 and F2 is less than that of F0 (Table 1). The opposite behavior was observed for O2 permeability, leading to a significant decrease (p < 0.05) of the values for FCD-containing films when these were compared with F0. Usually it is perceived that higher WVP would render higher O2 permeability;42 however, in this case, the radical scavenging functional groups of FCDs probably created additional sites in the film favoring the removal of oxygen, thereby minimizing the influence of adsorbed moisture in the film’s transport properties. Thus, the oxygen barrier property is improved due to the presence of FCDs.

Table 1. Physicochemical Properties of Pristine Protein (F0) and Nanocomposite Films (F1 and F2)a property

F0

F1

F2

thickness (μm) moisture content (% dry basis) WVP (ng m/m2 s Pa) oxygen permeability (ng day−1 Pa−1 m−1) tensile strength, TS (MPa) puncture strength, PS (N) elongation at break, EB (%) Hunter L Hunter a Hunter b opacity DPPH scavenging activity (%)

54.1 ± 0.2a 28.26 ± 1.64a

48.8 ± 0.15a 35.84 ± 1.09b

49.7 ± 0.09a 36.91 ± 1.38c

1.43 ± 0.62a 8.6 ± 0.3a

2.47 ± 1.56b 4.7 ± 0.2b

2.85 ± 1.42b 2.9 ± 0.2c

3.55 ± 0.51a

2.10 ± 1.78b

2.12 ± 1.15b

1.18 ± 0.03a

0.87 ± 0.01b

0.53 ± 0.09c

9.64 ± 0.02a

17.78 ± 0.69b

16.09 ± 1.03b

81.00 ± 0.13a 0.34 ± 0.01a 11.98 ± 0.12a 2.39 ± 0.17a 3.72 ± 1.61a

78.17 ± 0.16b 1.24 ± 0.03b 10.92 ± 0.07b 2.45 ± 0.13b 64.91 ± 2.83b

77.24 ± 0.09b 1.58 ± 0.05c 8.60 ± 0.04c 2.56 ± 0.02b 71.73 ± 4.01c

Results are the mean ± standard deviation of three replicates. Values followed by same letter within a row are not significantly different (p > 0.05).

a

TS, PS, and EB of a material are used to study their resistance to tensile stress, to determine the breaking force, and to express the percentage of increase in length that occurs before the sample breaks (i.e., film’s ability to stretch), respectively. The presence of FCDs in F1 and F2 influences their mechanical properties, decreasing the values of TS and PS (p < 0.05); on the other hand, an increase of EB values was observed. The reason behind the decreases in TS and PS by the addition of FCDs was the hydrophilic nature of C-dots, which favors the interaction with water molecules,42 as explained by the higher moisture content of the films. Due to its small size, FCDs fit easily into the polymer chains. Due to the hydrophilic groups of FCDs (especially −OH), it is likely to be strongly bonded by hydrogen bridges with glycerol and protein molecules especially at amine, amide, carboxyl, and hydroxyl sites.47 As a result, protein−protein interaction decreases and polymer segmental mobility increases (flexibility or EB enhances). Good optical properties of film are extremely important in food packaging for ensuring that consumers can clearly see the food product. The darkness of the FCD-loaded films was significantly higher (p < 0.05), as evidenced by the lower Hunter L values compared to the control. Meanwhile, in comparison with F0, increase in FCD concentration increased Hunter a (indicator of the tendency toward redness) and decreased Hunter b (indicator of the reduction of yellowness) values. F0 was more transparent (lower opacity value) than F1 and F2 (Table 1). The opacity of the film samples significantly increased (p < 0.05) with increasing FCD concentration. This observation could be explained by the fact that a higher amount of added FCDs hinders light passage through the film. This result accords with several published studies in which addition of antioxidants, polyphenols, plant extracts, or other additives to film matrices has been found to affect the optical properties severely. As the level of FCDs increased in the film formulation, so did the expected antioxidant character of the composite film (Table 1). In relation to F0, the DPPH scavenging activity increased by 17.5- and 19.3-fold in F1 and F2, respectively. F0 also showed 4517



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Figure 8. (a) Oil sample packed in F0, F1, and F2, as viewed under normal white light and UV light; changes in (b) free fatty acids (FFA), (c) peroxide value (PV), (d) conjugated dienes (CD), and (e) TBARS values of S0, S1, and S2 during storage.

some scavenging activity on DPPH. This is related to the fact that free radicals can react with the residual free amino (−NH2) groups to form stable macromolecular radicals, and the −NH2 groups can form ammonium (NH+3 ) groups by absorbing a hydrogen ion from the solution.26 It should be noted that although a high amount of FCDs was incorporated into the film, the FCDs in the protein film showed much lower antioxidant capacity than in the free form (aqueous dispersion) at much lower concentration (Figure 3e). This could be due to interaction between FCDs and the film matrix formed via hydrogen bonds and other cross-links, thereby reducing the availability of free structural factors such as hydroxyl, keto, and carboxylic groups,48 which imparts a scavenging feature to FCDs. Oxidative Stability Analyses of Rapeseed Oil Packed in Nanocomposite Films. On the basis of the above results, it was presumed that the use of FCD-incorporated composite packaging could be helpful in improving the oxidative shelf life of lipid food items during storage. As such, rapeseed oil, packed in small pouches made of either nanocomposite films (F1 and F2) or pristine protein film (F0), was monitored over time to assess the antioxidative effect of these films on a real food system. The samples packaged in nanocomposite film (S1 and S2) can easily be segregated from the control (S0) under UV light, on the basis of the inherent green luminescence of C-dots (Figure 8a). To consider the complexity of the lipid oxidation process, both the primary and secondary oxidation products have been assessed. Within the first 14 days of storage, FFA content increased perpetually in both S0 and S1 (p < 0.05); however, S2 showed negligible changes as compared to day 0 (p > 0.05) (Figure 8b). Subsequently, a gradual increase was found up to 21 days, and the highest FFA value was obtained on day 28. Possibly, lipid hydrolysis, catalyzed by lipases, occurred to a greater extent at the end of the storage period. On day 28, the respective FFA values of S1 and S2 were ∼1.2- and ∼1.4-fold less than that of S0. A small amount of FFA is usually noticeable in cold-pressed oil due to the release of enzymes from crushed seeds along with the oil. Alternatively, the accumulation of FFA could be

attributable to the lipases from micro-organisms, which was probably enhanced with extended storage.31 Unsaturated fatty acids react with molecular oxygen, usually via a free radical mechanism, to form hydroperoxides, the primary oxidation products.49 A marked increase in PV was observed in all of the tested samples for up to 14 days (p < 0.05) (Figure 8c). Thereafter, a decrease in PV was noticeable with extended storage (p < 0.05), which was presumed to be due to degradation of hydroperoxides, yielding a wide variety of secondary decomposition products including aldehydes. On day 28, the PV of S0 was almost 2 times higher than those of S1 and S2, which indicates the higher deterioration rate of S0 compared to the other samples. During oxidation of PUFAs containing methylene-substituted dienes and polyenes, there is a shift in the position of the double bond due to isomerization and conjugate bond formation (conjugated dienes).32 This is accompanied by increased UV absorption at 233 nm. It is an indicator of autoxidation and is reported to increase with uptake of oxygen and formation of peroxides, during the early stages of oxidation. Chaijan et al.31 reported that almost immediately after peroxides are formed, the nonconjugated double bonds (C C−CCC) that are present in natural unsaturated lipids are converted to conjugated double bonds (CC−CC). Thus, conjugated dienes are the primary oxidation products formed. A233 of S0 increased slightly up to 14 days of storage, after which its value showed a decreasing trend (Figure 8d). S1 and S2 exhibited similar profiles up to 21 days. From day 21 onward, A233 of S1 decreased and that of S2 continued to increase, until the end of the storage period (p < 0.05). This decrease probably occurred because of their decomposition into secondary oxidation products. Secondary lipid oxidation was studied by the TBARS value. Traditionally, the absorbance at 532 nm of the pink pigment formed in the reaction is measured. In all of the samples, the TBARS value increased sharply throughout the storage (p < 0.05) (Figure 8e). The marked increase in TBARS during 14− 28 days of storage was coincidental with the decrease in PV (Figure 8c). This was probably due to the destruction of hydroperoxides into relatively polar secondary products, 4518



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especially aldehydes in the later stages of lipid oxidation. It was most likely that a higher rate of lipid oxidation might be taking place at the end of the storage period (days 14−28). Additionally, the loss in natural antioxidants (such as tocopherol, polyphenols, etc.) of rapeseed oil during extended storage might contribute to the increased lipid oxidation. On the basis of the available data, it can be interpreted that the antioxidative FCDs in F1 and F2 retarded the lipid oxidation rate in S1 and S2 by quenching the free radicals. The formation of free radicals, which precedes lipid hydroperoxides, is the initial step in lipid oxidation.34 Abstraction of hydrogen atoms from allylic carbon atoms in unsaturated lipids generates a lipid radical, which in turn has the potential to initiate the chain reactions of the autoxidation cycle.49 The cycle is terminated either through a recombination of two radicals or by a reaction of the radical with antioxidant, leading to the formation of more stable radicals. The chain-breaking antioxidant, therefore, interrupts the oxidation chain reaction to enhance stability. Thus, FCD-incorporated packaging materials showed a profound impact on the oxidative shelf life of lipid product. Further studies need to be undertaken to improve the antioxidative and antimicrobial properties of these composite films. From this work it is concluded that high-value CNP can be easily synthesized from spent rapeseed meal by a simple green hydrothermal route. The obtained NP was found to be multifunctional, such as water solubility, high reductive potential, photoluminescence, and biocompatibility. This is due to the rich functionalities attached to the nanocarbons. The literature enumerates biopolymer-based films as good vehicles for the incorporation of active compounds; however, inclusion of fluorescent carbon nanostructures has rarely been evaluated. The present investigation shows the incorporation of FCDs into rapeseed protein film matrix, which adds new features to the film, for instance, photoluminescence, enhancement in antioxidant potential, and thermal stability. Moreover, oil samples packed in sachets made of FCD−protein composite film are able to resist oxidation better than those stored in pristine protein-based sachets. The work opens up new possibilities for agricultural residues as a valuable precursor of useful nanomaterials and can subsequently give rise to a new concept of biobased edible fluorescent food packaging, suggesting new scalable and simple approaches to improve environmental sustainability in industrial processes. Further studies should be undertaken to analyze the release behavior of FCDs from the film into different food model systems, their effect on organoleptic properties and the gastrointestinal tract, and toxicological assessment using in vitro and in vivo models.

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Article

AUTHOR INFORMATION

Corresponding Author

*(C.L.M.) Phone: +91 3712 267008 (5702). Fax: +91 3712 267005. E-mail: [email protected]. Funding

M.D.P. is grateful to DST-INSPIRE (India) and A.K.M. and V.K.D. are thankful to UGC-RGNF for meritorious fellowships. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the Research staff of the Department of Physics, Tezpur University, for valuable suggestions and PL study. We gratefully acknowledge Joston P. Nongkynrih and SAIF, NEHU, Shillong, for TEM imaging.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Optimal production of FCDs using response surface methodology (RSM), mechanism of C-dot formation from rapeseed spent meal, EDX spectrum of “oil-and-protein” spent meal, hemolytic activity of FCDs using 50 mg/mL concentration, standardization of the preparation of protein−FCD composite film, and biodegradation study of nanocomposite film. This material is available free of charge via the Internet at http:// pubs.acs.org. 4519



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